This invention relates to timers of the type advanced in a step-by-step manner by a drive mechanism and, more particularly, to the drive mechanism itself.
When designing a drive mechanism for a switching device in a program timer, a number of criteria must be taken into consideration. For example, it is desirable to advance the switching device at a relatively rapid rate for fast switching action. A common prior design is an indexing mechanism which drives the switching device in a step-by-step mannner and includes a ratchet track on the switching device and a drive pawl which cooperates with the ratchet track to index the switching device. A continuously rotating cam moves the drive pawl in a reverse direction against the action of a spring. When the camming surface drops off, the spring quickly moves the drive pawl in the forward direction to index the switching device. An improved mechanism is disclosed in U.S. patent application Ser. No. 616,092, filed June 1, 1984, now U.S. Pat. No. 4,536,626, wherein a drive pawl has a cam follower trapped between two camming surfaces of a drive cam. One of the camming surfaces defines the drive stroke of the drive pawl and the other camming surface defines the return stroke of the drive pawl. With the disclosed design, the switching time is about one fifth of the time between advances of the switching device. The switching time cannot be reduced substantially below this fraction because there is a limit to the slope of the camming surface defining the drive stroke.
It is therefore an object of this invention to provide a drive mechanism for advancing the switching device in a program timer in a step-by-step manner wherein the drive movement profile of the drive pawl is tailored by means of a camming surface and wherein the effective slope of the camming surface is increased so that the switching time is reduced with respect to the time interval between advancements.
In certain applications of program timers, it is desirable to be able to provide sub-interval switching. It is therefore a further object of this invention to provide such sub-interval switching in a program timer having a drive mechanism as described above.